Document QApplication::desktop as obsolete

There is no function equivalent in Qt 5.15, but the entire QDesktopWidget
class is already documented as obsolete, so this method should implicitly
no longer be used in new code.

Qt 6 lifts QScreen up to support the various QDesktopWidget use cases,
and a follow up will remove this method from Qt 6.0, or at least mark it
as deprecated API as well so that it
generates warnings when used.

Change-Id: I9b205e6d4a636c22a95728695088233c898cbfc4
Pick-to: 5.15
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/kernel/qapplication.cpp

@@ -2508,7 +2508,7 @@ void QApplicationPrivate::sendSyntheticEnterLeave(QWidget *widget)
 }
 
 /*!
-    \internal
+    \obsolete
 
     Returns the desktop widget (also called the root window) for \a screen.